The role of residual stresses in layered composites of y-ZrO2 and A120 16522 (a) Position across the layer, um Fig 9. Frequency shift of the RI line and compressive stresses 508 in 25 um thick alumina layer of Y-ZrO2/Al2O3 composite as a function of position across the layer. stress minimum in the center of thicker alumina layers can be the result of internal relaxation of stresses The compressive stress gradient( the difference of 16524 stress between the layer boundary and the center of SA 35/ the layer) listed in Table 2 can be correlated with e85° the angles of crack deflection. Good correspon dence indicates that the gradient can be regarded an important factor responsible for the degree of crack deflection and further contribution of crack Fig. 7. The crack path in alumina layer of Y-ZrO2/Al2o deflection mechanism in enhancing the toughness composite at the crack front ( inverted image) observed in layered composites(see Table 3) To the stress distribution shown at Fig. 3 one new stress component should be added however. Finite element calculations 2, 3 show that in layered materials not only biaxial stresses exist at the surface and far from the surface. but a stress perpendicular to the layer plane existing near the free surface that is highly localized, decreasin rapidly face to becom dligible distance approximately on the order of the layer 1o thickness, also. This stress has a sign opposite to Position acros that of the biaxial stresses deep within the layer Thus, when the biaxial stresses are compressive 250 there is a tensile stress perpendicular to the layer at and near the surface. This reversal of stresses was also observed by Cox 4 during his analysis of inclusions located either within a body or at the surface. Thus. a tensile stress field. localized near the surface, will be present in layers when the stress o far from the surface is biaxial compressive. These tensile stresses can cause the extension of preexist- Fig8. Frequency shift of the Ri line and compressive stresses ing cracks. Such a cracks along the center of the in 10 um thick alumina layer of Y-ZrO2/Al2O3 composite as a two phase Al2O3/3Y-ZrO2 layer(300 um) bonded function of position acro by two much thicker (3000 um)3Y-ZrO2 layersstress minimum in the center of thicker alumina layers can be the result of internal relaxation of stresses. The compressive stress gradient (the di€erence of stress between the layer boundary and the center of the layer) listed in Table 2 can be correlated with the angles of crack de¯ection. Good correspon￾dence indicates that the gradient can be regarded as an important factor responsible for the degree of crack de¯ection and further contribution of crack de¯ection mechanism in enhancing the toughness observed in layered composites (see Table 3). To the stress distribution shown at Fig. 3 one new stress component should be added however. Finite element calculations12,13 show that in layered materials not only biaxial stresses exist at the surface and far from the surface, but a stress perpendicular to the layer plane existing near the free surface that is highly localized, decreasing rapidly from the surface to become negligible at a distance approximately on the order of the layer thickness, also. This stress has a sign opposite to that of the biaxial stresses deep within the layer. Thus, when the biaxial stresses are compressive, there is a tensile stress perpendicular to the layer at and near the surface. This reversal of stresses was also observed by Cox14 during his analysis of inclusions located either within a body or at the surface. Thus, a tensile stress ®eld, localized near the surface, will be present in layers when the stress far from the surface is biaxial compressive. These tensile stresses can cause the extension of preexist￾ing cracks. Such a cracks along the center of the two phase Al2O3/3Y±ZrO2 layer (300m) bonded by two much thicker (3000m) 3Y±ZrO2 layers Fig. 7. The crack path in alumina layer of Y±ZrO2/Al2O3 composite at the crack front (inverted image). Fig. 8. Frequency shift of the R1 line and compressive stresses in 10 m thick alumina layer of Y±ZrO2/Al2O3 composite as a function of position across the layer. Fig. 9. Frequency shift of the R1 line and compressive stresses in 25m thick alumina layer of Y±ZrO2/Al2O3 composite as a function of position across the layer. The role of residual stresses in layered composites of Y±ZrO2 and Al2O3 259